This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Based on our previous high-field/high-frequency EPR work on protein systems , we will focus on orientation-resolved pulsed high-field EPR spectroscopy and related double-resonance techniques applied to wild type and site-specific mutants of photosynthetic organisms, in particular the reaction centers (RCs) from non-oxygenic purple bacteria and photosystems I and II (PS I, PS II) from oxygenic cyanobacteria and algae. The mutant RC preparations will be Fe-to-Zn substituted and studied by advanced multifrequency/multiresonance EPR techniques (operating at the MPI and at ACERT) in frozen-solution samples. The experimental methods used so far are planned to be extended by or combined with the unique instrumental methods available at ACERT, which are largely complementary to the EPR facilities at the MPI M[unreadable]lheim. This option, in conjunction with the immense experience in designing and constructing novel EPR instrumentation at ACERT, will result in innovations and technological advances beyond what is presently known in bio-oriented EPR spectroscopy. As an example, it is planned to combine the ACERT expertise in stopped-flow EPR in the 8-18 GHz range with our preliminary fast-flow EPR experiments at 95 GHz.